In general fruit size in tomato is inherited as a quantitative trait, meaning that there are many genes acting to determine the fruit size. Current estimates are that at least 10 genes have significiant effects and of those six genes are important. With modern techniques geneticists are able to identify and map those genes.

When large-fruited tomato plnnts are crossed with small-fruited tomato plants the small size predominates in the F1. A cross of a plant with fruit weighing 1.1 g with one whose fruit weighed about 500 g produced F1 offspring whose fruit weighed on average 10.5 g and the F2 generation had fruit weighing an average of 11.1 g.

From "Dissecting the Genetic Pathway to Extreme Fruit Size in Tomato Using a Cross Between the Small-Fruited Wild Species Lycopersicon pimpinellifolium and L. esculentum var. Giant Heirloom" Genetics, Vol. 158, 413-422, 2001

Here is a link that might be useful: Inheritance of fruit size in tomato

Also worth mentioning that in any crosses where you're talking about multi-gene traits, you can get a hybrid with trait values outside either parental range--i.e., smaller than the small tomato or larger than the large tomato.

Patrick Alexander

An F2 generation plant is more likely to be outside of the range of the two parents than an F1 generation plant. Very often the F1 generation is less variable than either parent and the variability increases in the F2 generation.

Also, it is more probable that the F2 generation will have individuals that are outside of the parental range if the parents are more similar in size. A cross of a 500g fruit weight variety with a 300g fruit weight variety is more likely to produce F2 offspring with fruit weights over 500g or under 300g than a cross of a 500g fruit weight variety with a 1g fruit weight variety is to produce F2 offspring with weights over 500g or under 1g.

Even though the frequencies of such offspring would differ among crosses (the frequencies will depend on the genetic, environmental and interaction effects for both the means and variances of fruit weight), if enough F2 offspring are produced one could find some that have weights outside of the parental range.

A study of fruit size in the F1 of tomato crosses was written up in the newsletter of the Tomato Genetics Cooperative. The F1 fruit were the geometric mean of the parents.
The geometric mean is found by multiplying two numbers and taking the square root of the product.
This is in contrast to the arthimatic mean where you add two numbers and divided by two.
The geometric mean is always smaller than the arthimatic mean.
The greater the difference between the two numbers, the greater the difference between the arthimatic and the geometric means.

The likelyhood of getting segregants larger than either in the F2 tends to encrease as the degree of relatedness of the parental lines decreases.

Walter, was that a measure of wieght or diameter? The two measures are not linear in proportion.

The F2 would have number of weights(mass) and there would be some fruits like their parents and many more with numerous shapes and sizes.Peppers have more than 20 genes controling fruit shape and size.The tomato should have close to same amout of genes.peppers and tomatoes are very simalar (same family)

"In general fruit size in tomato is inherited as a quantitative trait, meaning that there are many genes acting to determine the fruit size."

Are cherry tomatoes simply small in size as a result of a quantitative trait or are they a distinctly different type than large fruited tomatoes? Are there intermediate varieties?

Jim

"Are cherry tomatoes simply small in size as a result of a quantitative trait?" Yes.

"Are there intermediate varieties?" In size? One creates intermediates when a cherry tomato variety is crossed with a large sized tomato variety. For example, in a cross of a cherry tomato variety (fruit fresh weight 3.7 g) with a 'large-fruited' variety (68.7 g) the fruit of the F1 plants averaged 10.7 g and the fruit of the back-cross plants (to the large-fruited parent) averaged 19.8 g [Euphytica 176:137-147, 2010].